Several reports have been published describing the presence of volatile N-nitrosamines in various rubber products. The present concern about the occurrence of volatile N-nitrosamines in baby bottle rubber nipples and the possible migration of these compounds into infant formula was prompted by a report of Preussmann et al., (1981) Am. Chem. Soc. Symp. Ser. 174, American Chemical Society, Washington, D.C., p. 217.
A method was described for the estimation of volatile N-nitrosamines in the rubber nipples of baby bottles. In a study of rubber nipples from one manufacturer, N-nitrosodimethylamine, N-nitrosodiethylamine and N-nitrosopiperidine were determined by gas chromatography, using a thermal energy analyzer, and their presence was confirmed by mass spectrometry with average levels of individual nitrosamines ranging from 22 to 281 ppb. When the nipples were sterilized in a conventional sterilizer together with milk or infant formula the three nitrosamines migrated into the milk or formula. Storing a bottle of milk with a rubber nipple inverted in it for 2 hr at room temperature or overnight in a refrigerator after sterilization resulted in an 8-13% average increase in the nitrosamine levels migrating into the milk. On repeated sterilization of a single nipple, the quantities of nitrosamines migrating into milk from rubber nipples declined steadily, but after seven sterilizations, nitrosamines were still readily detectable in the milk. Nitrosamine levels were higher in rubber nipples after sterilization, indicating the presence of nitrosamine precursors in the nipples. No nitrosamines were found in raw, uncured rubber. Chemical accelerators and stabilizers added during the organic rubber vulcanization process are the source of the amine precursors in rubber nipples.
On Jan. 1, 1984, the U.S. Food and Drug Administration (hereinafter "FDA") established an action level of 60 ppb total N-nitrosamines in rubber nipples. The action level was reduced to 10 ppb on Jan. 1, 1985.
A collaborative study was conducted on the FDA dichloromethane extraction method for determining volatile N-nitrosamines in baby bottle synthetic and natural organic rubber nipples. Following dichloromethane extraction, N-nitrosamines were determined by gas chromatography-thermal energy analysis. Six pairs of blind duplicate rubber nipple samples representing 6 lots were analyzed by 11 collaborating laboratories. All samples were portions taken from equilibrated composites of cut-up rubber nipples obtained from manufacturers in the United States. Recoveries of the internal standard (N-nitrosodipropylamine) at approximately 20 ppb ranged from 10 to. 120%. Reproducibility relative standard deviations (RSD) were between 35 and 45% for N-nitrosamine levels from 10 to 20 ppb. However, when data from laboratories with recoveries less than 75% were excluded (this is now specified in the method), RSD, values were between 11 and 32% for N-nitrosamine levels from 6 to 26 ppb. Values were consistent with or better than those reported for other analytical techniques designed to quantitate trace contaminants at the low ppb level, e.g., aflatoxin in food. The method has been adopted as an official first action for the quantitation of volatile N-nitrosamines in baby bottle rubber nipples. See Gas Chromatographic-Thermal Energy Analysis Method for Determination of Volatile N-Nitrosamines in Baby Bottle Rubber Nipples: Collaborative Study, by Gray & Stachiw, J. Assoc. Off. Anal. Chem. (1987) 70, March Issue.
Various additives have been reported to prevent nitrosamine formation. In one case the order of addition of reagents was found to be key to prevention of nitrosamine formation in rubber and the additive erythorbic acid was employed. T. Fukuda, Y. Matsunra, S. Kusumoto, Nippon Shokuhin Kogyo Gakkaishi, 28 (1981) 606, CA 96, 85000e. Sorbic acid and sodium ascorbate were reported to be effective at preventing nitrosamine formation. See K. Tanaka, K. C. Chung, H. Hayatsu, T. Kada, Food Cosmet. Toxicol., 16 (1978) 209 and R. L. S. Patterson, D. S. Mottram, Abstr. Commun. Eur. Meet. Meat Res. Work., 20th An Foras Tuntais: Dublin, Ire. (1974) 77, CA 85, 19272s. Alcohols in general inhibit nitrosamine formation at pH&lt;7 while hindered alcohols such as butylated hydroxy anisole (BHA) was reported to inhibit the formation of nitrosamine under conditions of oxidation of amines by peroxide. See T. Kurechi, K. Kikugawa, T. Kato, Food Cosmet. Toxicol., 18 (1980) 591, CA 94, 191596x and N. S. Shehad, PCT Int. Appl. WO 9322273, CA 120, 191111r.
Although research in the synthetic and natural organic rubber industry has been devoted to lowering or eliminating nitrosamines, none of these studies have included silicone rubber materials. Silicone elastomeric compositions, in contrast to synthetic and natural organic rubber compounds, can be prepared from a vinyl-containing polydiorganosiloxane, an organohydrogensilicone crosslinker, and a platinum catalyst. The compositions of this type are desirable for many reasons. For instance, they cure without by-products. They can cure at room temperature or at elevated temperatures. They can be stabilized for storage at room temperature by utilization of a platinum catalyst inhibitor. They can be made from high and low viscosity polymers. These compositions utilize components that are low enough in viscosity that they are easily pumpable or extrudable as well as have a fast cure time. These compositions also provide cured silicone elastomers which are characterized by high strength and high durometer. Thus, these silicones became leading candidates to replace the organic rubber compounds.
Analysis of the cured silicone elastomers showed no presence of nitrosamines. However, upon post-baking as required by FDA and European BGA, the presence of nitrosamines was detected. When the cured sheet is post-baked, open (PBO) at 200.degree. C., 3-7 ppb dimethylnitrosamine (DMNA) is detected. If the sheet is first wrapped in aluminum foil and then post baked, sealed, (PBS) the sheets had from 30-70 ppb DMNA.
For silicones to serve these FDA regulated markets, Applicants developed various techniques to prevent formation of nitrosamine during post bake. In co-pending application Ser. No. 08/298,981 filed Aug. 31, 1994, applicants indicated that post baking in an atmosphere substantially free of oxygen can prevent formation of nitrosamine during post baking. In another co-pending application Ser. No. 08/298,728 filed Aug. 31, 1994, applicants reported that treating the polymer/filler mixture with an effective amount of acid can also prevent the formation of nitrosamine. Although these methods have been proven to be effective, not all post baking facilities are equipped with inert atmosphere capacities. The use of acid can be corrosive to some equipment. Therefore, applicants continued to look for a new method.